Disable lock screen notification by default

This is reland of crrev.com/c/1206551 after fixing the memory leak
(crrev.com/c/1208694). The diff is completely same as the previous CL.
The previous CL had been reverted due to the ASAN failure (see.
crbug.com/880227).

[Original description]
The feature is not launched on M70. This CL should be merged to M70 branch as well.

TBR: yamaguchi@chromium.org
TBR: oshima@chromium.org
Bug: 880227
Change-Id: Id0ca9b5ee488130d7fb0c414b4a9522117d462dd
Reviewed-on: https://chromium-review.googlesource.com/1212902Reviewed-by: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589485}

ash/public/cpp/ash_features.cc

@@ -26,7 +26,7 @@ const base::Feature kKeyboardShortcutViewerApp{
     "KeyboardShortcutViewerApp", base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kLockScreenNotifications{"LockScreenNotifications",
-                                             base::FEATURE_ENABLED_BY_DEFAULT};
+                                             base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kLockScreenInlineReply{"LockScreenInlineReply",
                                            base::FEATURE_DISABLED_BY_DEFAULT};